There is increasing concern regarding the hazards which may result from exposure of humans to electromagnetic energy. A large number of electromagnetic radiators, including radio and television stations, radar transmitters, microwave ovens, and communications equipment, contribute to the total exposure of humans to non-ionizing radiation. In addition, some industrial workers are exposed to the fields from such devices as radio frequency heat sealers and inductive heaters. Exposure to the field of EHV lines used for power transmission is also becoming more common. The National Institute for Occupational Safety and Health (NIOSH) and the American National Standards Institute (ANSI) have both been involved in attempting to define safety criteria.
Considerable effort is being spent in both experiments with animal models and theoretical analysis to examine the biological effects of non-ionizing electromagnetic energy. An important part of this work is dosimetry, that is determining the amount of energy absorbed under various conditions. Dosimetry is much more difficult for nonionizing electromagnetic energy than it is for ionizing radiation because, unlike ionizing radiation, the absorbed dose is not simply related to the incident flux. The absorption of electromagnetic energy is dependent upon the dimensions, composition and posture of the body, as well as the frequency, polarization, and other properties of the radiations. Generally the most intense exposure occurs under near-field conditions, where the electric and magnetic fields are not simply related and where it is often difficult to accurately characterize the source.
There is a strong need for a non-invasive personal dosimeter for electromagnetic energy corresponding to the devices that are in general use with ionizing radiation; however, no suitable non-invasive device has thus far been suggested. Dosimetry for electromagnetic energy has been limited to invasive measurements of the temperature or fields within laboratory animals and models of man, as well as to computer simulations. The fields external to the body are a superposition of incident and scattered waves, so that measurements made close to the body generally fail to permit accurate characterization of either the external fields or those of the incident wave. For this reason, it has generally been impossible to make meaningful predictions of the absorbed dose from non-invasive measurements.
More specifically, there is also considerable need for a device which would quantify the dose received by a patient when electromagnetic energy is used for therapeutic purposes, such as diathermy and the hyperthermic treatment of cancer. Hyperthermia has shown considerable promise for the adjuvant treatment of cancer, but there has been difficulty in treating deep-seated tumors with the required degree of precision. Computer simulations and clinical observations suggest that considerable aberrant heating may occur in hyperthermia due to deposition of energy outside the region intended for treatment. At present, it is only possible to monitor the temperature at a few intracavity or interstitial locations, so that the physician must rely on the complaints of pain from the patient for guidance during treatment. However, heating at locations in the core of the body is often perceived as a dull pressure, so patient complaints are difficult to interpret, but significant damage can still occur.
Ampere's Law states that the line integral of the magnetic field intensity around the closed path is equal to the total electric current passing through the region enclosed by the path. EQU H.multidot.dl=I (1)
A ferromagnetic core having cross-sectional area A and permeability .mu., may be formed to make a closed loop of length L. If the core has a high permeability, the flux within the core at any part of the loop is related to the current passing through the aperture enclosed by the loop, according to the following equation: EQU .PHI.=.mu.AI/L (2)
A coil consisting of N turns may be wound around the core at any location on the loop and the potential induced on the coil is given by the equation: EQU V=-jw.mu.NAI/L (3)
where time dependence of e.sup.jwt is assumed.
Equation 3 can be used to determine the current I from the potential V measured across the coil. This principle has been used in clamp-on ammeter instruments for many years. These devices allow the measurement of a.c. current in power circuits without cutting the lines and interrupting service. Current probes are also commercially available, which serve as transducers to be used with separate meters, oscilloscopes, or other measuring instruments. These probes have either a clamp-on design, which opens for placement around the conductor, or a fixed configuration which requires that the conductor be passed through the central aperture.